Hypertension, or high blood pressure, is a major cardiovascular disorder that is estimated to affect over 50 million people in the United Sates alone, and is a leading cause of heart failure and stroke. It is the primary cause of death in over 42,000 patients per year and is listed as a primary or contributing cause of death in over 200,000 patients per year in the United States alone. Hypertension occurs in part when the body's smaller blood vessels (arterioles) constrict, causing an increase in blood pressure. Because the blood vessels constrict, the heart must work harder to maintain blood flow at the higher pressures. Sustained hypertension may eventually result in damage to multiple body organs, including the kidneys, brain, eyes and other tissues, causing a variety of maladies associated therewith. The elevated blood pressure may also damage the lining of the blood vessels, accelerating the process of atherosclerosis and increasing the likelihood that a blood clot may develop. This could lead to a heart attack and/or stroke.
Sustained high blood pressure may eventually result in an enlarged and damaged heart (hypertrophy), which may lead to heart failure. Heart failure is the final common expression of a variety of cardiovascular disorders, including ischemic heart disease. It is characterized by an inability of the heart to pump enough blood to meet the body's needs and results in fatigue, reduced exercise capacity and poor survival. It is estimated that approximately 5,000,000 people in the United States suffer from heart failure, directly leading to 39,000 deaths per year and contributing to another 225,000 deaths per year.
Heart failure results in the activation of a number of body systems to compensate for the heart's inability to pump sufficient blood. Many of these responses are mediated by an increase in the level of activation of the sympathetic nervous system, as well as by activation of multiple other neurohormonal responses. Generally speaking, this sympathetic nervous system activation signals the heart to increase heart rate and force of contraction to increase the cardiac output; it signals the kidneys to expand the blood volume by retaining sodium and water; and it signals the arterioles to constrict to elevate the blood pressure. The cardiac, renal and vascular responses increase the workload of the heart, further accelerating myocardial damage and exacerbating the heart failure state.
Heart failure can be generally classified into two categories: systolic and diastolic heart failure. The heart contracts and relaxes with each heartbeat—these phases are referred to as systole (the contraction phase) and diastole (the relaxation phase). Systolic heart failure (SHF) is characterized by low ejection fraction. In patients with diastolic heart failure (DHF), contraction may be normal but relaxation of the heart may be impaired. This impairment is generally caused by a stiffening of the ventricles. Such impairment is referred to as diastolic dysfunction and if severe enough to cause pulmonary congestion (increased pressure and fluid in the blood vessels of the lungs), diastolic heart failure. DHF patients differ from those patients with SHF, in that DHF patients may have a “normal” ejection fraction. However, because the ventricle doesn't relax normally, the pressure within the ventricle increases and the blood filling the ventricle exceeds what is “normal”. People with certain types of cardiomyopathy may also have diastolic dysfunction.
Left ventricular hypertrophy refers to a thickening of the left ventricle as a result of increased left ventricular load. Left ventricular hypertrophy can be a significant marker for cardiovascular disorders and most common complications include arrhythmias, heart failure, ischemic heart disease, and sudden death. Although left ventricular hypertrophy (LVH) increases naturally with age, it is more common in people who have high blood pressure or have other heart problems. Because LVH usually develops in response to hypertension, current treatment and prevention mainly includes managing hypertension. Typical diagnosis involves the use of echocardiograms (ECHO) and electrocardiograms (ECG).
The SphygmoCor system (AtCor) provides a non-invasive assessment of the cardiovascular system and autonomic function. The SphygmoCor waveform provides physicians with clinically important information using a variety of cardiovascular parameters including augmentation index, augmentation pressure, central pulse pressure, central systolic pressure, and ejection duration. The SphygmoCor system comprises a non-invasive pressure transducer that uses a radial artery (external wrist) measurement and BP monitor. It works by using the pressure probe to record the pressure wave at the radial artery, which is then calibrated with the brachial blood pressure. However, this system does not provide a direct measurement of pressure, and the system is not able to be used as an implanted, continuous system.
Vascular stiffness in aging and its relation to cardiovascular disease is an important topic of current research. Vascular stiffness has been proposed as a risk factor for overall cardiovascular morbidity and mortality because of its suggested role in elevated blood pressure, increased left ventricular mass and heart failure. Current therapies targeting vascular stiffness, therefore, include those prescribed for these conditions, such as hypertension drug therapy. Pulse pressure is a known, simple measurement that can be used as a surrogate to aortic stiffness. Additionally, wave reflections can be inferred from detailed computer-aided pulse wave contour analysis, such as with the AtCor SphygmoCor system.
Heart failure is known to be a multi-organ disease and there is recent evidence that the gut (including the splanchnic circulation) plays an important role in cardiac diseases. The circulatory system of humans includes the pulmonary circulation and the systemic circulation. The pulmonary circulation ensures deoxygenated blood is returned to the lungs and oxygenated blood returned to the heart, while the systemic circulation ensures that oxygenated blood is supplied to the body. The systemic circulation includes the splanchnic circulation (or visceral circulation), which ensures that digestive organs receive blood through the vessels supplying the abdominal viscera. Acute decompensated heart failure or pulmonary congestion develops as a result of blood being redistributed from the splanchnic circulation to the pulmonary circulation, manifesting in fluid build-up in the chest and an inability of the patient to breathe. The specific mechanism of this transfer of fluids between the two is not yet fully understood.
The current standard of care for a patient exhibiting congestion is a variety of drugs, including diuretics, which causes excretion of fluid through the renal system. Splanchnic nerve stimulation has been described in the art to treat shock. For example, WO 2006/0031902 to Machado et. al teaches a method of treating hemodynamic derangement and controlling the mobilization of splanchnic circulation by stimulating the splanchnic nerve. Neither drug therapy, nor nerve stimulation therapy has been successful in providing a controlled therapy for congestive heart failure or acute decompensated heart failure.
A number of drug treatments have been proposed for the management of hypertension, heart failure, and other cardiovascular disorders. These include vasodilators to reduce the blood pressure and ease the workload of the heart, diuretics to reduce fluid overload, inhibitors and blocking agents of the body's neurohormonal responses, and other medicaments. Such medications can be effective for a short time, but cannot be used for expended periods because of side effects. Various surgical procedures have also been proposed for these maladies. For example, heart transplantation has been proposed for patients who suffer from severe, refractory heart failure. Alternatively, an implantable medical device such as a ventricular assist device (VAD) may be implanted in the chest to increase the pumping action of the heart. Alternatively, an intra-aortic balloon pump (IABP) may be used for maintaining heart function for short periods of time, but typically no longer than one month.
Each of these approaches may be at least partly beneficial to patients, however each of the therapies has its own disadvantages. For example, drug therapy is often incompletely effective. Drugs often have unwanted side effects and may need to be given in complex regimens. These and other factors contribute to poor patient compliance with medical therapy. Drug therapy may also be expensive, adding to the health care costs associated with these disorders. Likewise, surgical approaches are very costly, may be associated with significant patient morbidity and mortality and may not alter the natural history of the disease.
Accordingly, there continues to be a need for improved devices and methods for diagnosing, treating and/or managing high blood pressure, heart failure, and their associated cardiovascular and nervous system disorders.